1. Field of the Invention
The present invention relates to an electrolytic membrane fuel cell, in particular to a structure of the electrolytic membrane fuel cell.
2. Related Art
A fuel cell or electrochemical cell, which generates an electric power by virtue of an oxidation/reduction reaction through an electrolyte, is utilized for various applications. For this purpose, electrodes are disposed at opposite sides of such electrolyte and a reactive gas are introduced thereto to generate the electric power. As a configuration of the fuel cell, a polymer electrolyte fuel cell(PEFC) or a solid PEFC has been known. Generally, the solid polymer electrolyte fuel cell is formed by laminating a power generating element or solid polymer electrode bonded assembly in which a solid polymer electrolyte membrane with hydrogen ion conductivity is sandwiched between a pair of platinum (Pt) electrode catalyst deposition carbon electrodes and gas separators which are disposed at opposite sides of the bonded assembly for supporting the assembly, and are formed with grooves to which the reactive gas is supplied. With this structure, a fuel gas is introduced to one of the electrodes and an oxidant gas is introduced to the other electrode so that an electrochemical energy due to the fuel gas and oxidant gas is directly converted to an electric energy to obtain the electric energy.
In the solid polymer electrolyte fuel cell, when an electrochemical reaction occurs between the hydrogen and oxygen, an electrical current is generated between the electrodes while water is produced as by-product in the cathode. The operative temperature of the solid polymer electrolyte fuel cell is as low as about 80.degree. C. compared with other types of fuel cells. Thus, the polymer electrolyte fuel cell is suitable for a portable power source, specifically, a power source for electric automobile.
However, it should be noted that the automobile is necessary to find a supply source of the hydrogen gas as a fuel gas in its own facility, such as a portable tank, reformer or the like. On the other hand, an ambient air is used as the oxidant gas by reason of weight, cost of the system and the like. In this case, since the air is of one-fifth oxygen density of a pure oxygen, a problem occurs in terms of an oxidation/reduction reaction speed, and mass transfer speed during the reaction in the fuel cell.
In view of the above problem, the air is generally compressed to be introduced to the fuel cell. It should, however, be understood that an energy efficiency in the fuel cell as whole is lowered because it cannot help consuming a certain amount of energy for driving an air compressor.
Under the circumstances, various ways have been proposed that an enhanced energy efficiency can be accomplished under a low partial pressure of oxygen.
For example, it has been known that an electrocatalyst substance (usually a platinum which is active for the oxidation/reduction reaction under a low temperature condition as low as 80.degree. C.) is atomized to improve an electrocatalyst activity, that the electrocatalyst substance is carried by a corrosion resistant carbon to improve a catalyst adsorption, that a platinum sputtering deposition is made in order to concentrate anode gas, cathode gas in a three boundary phase area among ion conductor, electrocatalyst and reactive gas to thereby improve the electrocatalyst activity, and the like.
In this case, the platinum sputtering deposition is made on a surface of the electrocatalyst layer so that a surface area of the electrocatalyst for exerting the electrochemical property thereof is increased to improve the oxygen reduction reaction (ORR) kinetics.
It should, however, be noted that there are problems in making the platinum sputtering deposition as follows. Namely, the platinum sputtering deposition formed on the surface of the polymer electrolyte membrane of the electrode covers a surface of the electrocatalyst to impede a transfer of water and the like resulting in degradation of the energy efficiency of the fuel cell as a whole.
Japanese Un-examined Patent publication No. 7-134995 discloses that water produced around the cathode is effectively removed to improve fuel cell performance. In the above publication, the fuel cell includes a solid polymer electrolyte membrane and a fuel electrode and air electrode disposed at opposite sides of the electrolyte membrane. An electrocatalyst layer of the air electrode includes carbon particles with hydrophobic surface and electrocatalyst carried by the carbon particles. An electrocatalyst layer of the fuel electrode includes carbon particles with hydrophilic surface and electrocatalyst carried by the carbon particles. The above publication states that with this structure, both problems of a flooding of the by-product water around the air electrode and water shortage in the electrolyte close to the fuel electrode can be eliminated.
The above Japanese publication No. 7-134995 provides a fuel cell structure which can effectively control the supply and discharge of the water around the anode and cathode. Thus, this type of fuel cell can improve the fuel cell performance in terms of mass transfer in the cell. However, this does not take totally account of both the electrocatalyst activity and mass transfer. Therefore, there is a certain limit in improvement for the fuel cell performance.